Originální popis anglicky:
random, urandom - kernel random number source devices
The character special files
/dev/random and
/dev/urandom (present
since Linux 1.3.30) provide an interface to the kernel's random number
generator. File
/dev/random has major device number 1 and minor device
number 8. File
/dev/urandom has major device number 1 and minor device
number 9.
The random number generator gathers environmental noise from device drivers and
other sources into an entropy pool. The generator also keeps an estimate of
the number of bits of noise in the entropy pool. From this entropy pool random
numbers are created.
When read, the
/dev/random device will only return random bytes within
the estimated number of bits of noise in the entropy pool.
/dev/random
should be suitable for uses that need very high quality randomness such as
one-time pad or key generation. When the entropy pool is empty, reads from
/dev/random will block until additional environmental noise is
gathered.
When read,
/dev/urandom device will return as many bytes as are
requested. As a result, if there is not sufficient entropy in the entropy
pool, the returned values are theoretically vulnerable to a cryptographic
attack on the algorithms used by the driver. Knowledge of how to do this is
not available in the current non-classified literature, but it is
theoretically possible that such an attack may exist. If this is a concern in
your application, use
/dev/random instead.
If your system does not have
/dev/random and
/dev/urandom created
already, they can be created with the following commands:
mknod -m 644 /dev/random c 1 8
mknod -m 644 /dev/urandom c 1 9
chown root:root /dev/random /dev/urandom
When a Linux system starts up without much operator interaction, the entropy
pool may be in a fairly predictable state. This reduces the actual amount of
noise in the entropy pool below the estimate. In order to counteract this
effect, it helps to carry entropy pool information across shut-downs and
start-ups. To do this, add the following lines to an appropriate script which
is run during the Linux system start-up sequence:
echo "Initializing kernel random number generator..."
# Initialize kernel random number generator with random seed
# from last shut-down (or start-up) to this start-up. Load and
# then save 512 bytes, which is the size of the entropy pool.
if [ -f /var/random-seed ]; then
cat /var/random-seed >/dev/urandom
fi
dd if=/dev/urandom of=/var/random-seed count=1
Also, add the following lines in an appropriate script which is run during the
Linux system shutdown:
# Carry a random seed from shut-down to start-up for the random
# number generator. Save 512 bytes, which is the size of the
# random number generator's entropy pool.
echo "Saving random seed..."
dd if=/dev/urandom of=/var/random-seed count=1
The files in the directory
/proc/sys/kernel/random (present since 2.3.16)
provide an additional interface to the
/dev/random device.
The read-only file
entropy_avail gives the available entropy. Normally,
this will be 4096 (bits), a full entropy pool.
The file
poolsize gives the size of the entropy pool. Normally, this will
be 512 (bytes). It can be changed to any value for which an algorithm is
available. Currently the choices are 32, 64, 128, 256, 512, 1024, 2048.
The file
read_wakeup_threshold contains the number of bits of entropy
required for waking up processes that sleep waiting for entropy from
/dev/random. The default is 64. The file
write_wakeup_threshold
contains the number of bits of entropy below which we wake up processes that
do a
select() or
poll() for write access to
/dev/random.
These values can be changed by writing to the files.
The read-only files
uuid and
boot_id contain random strings like
6fd5a44b-35f4-4ad4-a9b9-6b9be13e1fe9. The former is generated afresh for each
read, the latter was generated once.
/dev/random
/dev/urandom
The kernel's random number generator was written by Theodore Ts'o
(tytso@athena.mit.edu).
mknod (1)
RFC 1750, "Randomness Recommendations for Security"
